Channel service unit

A channel service unit (CSU) is a DCE used to support communication over a digital services circuit. Located on customer premises, the CSU terminates the electrical connection, provides for equalization (to deal with distortion), supports loopback capabilities for testing, and generates signals to keep the network connection alive even if the attached customer equipment is not functioning.

User Interface Equipment

The CSU performs many functions necessary to maintain and use T-1 network connections. First, it provides a termination point for the four wire copper facilities. Second, it ensures that T-1 terminating equipment complies with T-1 line code specifications, such as ones density, bipolar AMI, proper voltage formats (+3 and -3 volts), and pulse width requirements. Other CSU functions include diagnostic and customer loopback capability, which are maintenance and test functions. Another important function provided by a CSU is generation of a “keep alive” signal that allows the network connection to be maintained when the terminating equipment is disconnected or not functioning. The CSU is customer-provided equipment and is powered locally by the customer power supply, not the telco.

Modern T-1 terminating equipment (e.g., PBX, router, frame relay access device, or multiplexer) often has the CSU function built into the CPE. However, upgrades to OAM&P functions provided by the networks might be more difficult and costly to deploy than for individual stand-alone or rack mounted plug-in CSUs.